React. Kinet. Ca~al. Lett., Vol. 27, No. 1, 77~81 (1985)

KINETICS OF HYDROGEN ADSORPTION ON THIN IRON FILMS WITH PREADSORBED HYDROGEN

SULFIDE

P. Nowacki* [ , W. Lisowski and R. Dug

Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warszawa, Poland

* Space t~eseareh Centre, Polish Academy of SCiences, ul. Ordona 21, 01-237 Warszawa, Poland

l~eeeived March 5, 1984 Accepted April 18, 1984

The influence of H~S preadsorbed on thin iron filnq surface on the character of TD spectra and on sticking probability dependence on population for hydrogen adsorption was studied. Kisliuk's model for adsorption with the precursor state was examined.

IdCCJ~e/IOBaH~ Boag~Rge npe~aneop6u~u H2S Ha TOHKHX HHeHKax ~<eneaa Ha xa- pa~Tep cneKTpOB Tep-~oaecop6uHu H Ha 3aBHCHM0CTI~ I<OS~qbI4RIleltTa r~puHx4na- H~H OT n0nya~unu npn nocne~y~ome~ ~ecop6~nn BO~0p0~Ia. HpoBe~eHa nonb~TKa HpHMeHeHu~ ~.t~ /laHHO~ CHCTeMN MOReJI~ t<~camKa, onHcNBamule~ ancop6uHm c npeKypcopHb~M C0CTOSHHeM.

The aim of this work was to give some informat ion on the inf luence of H2S p readsorbed on iron surface on the kinetics of hydrogen adsorpt ion.

The studies were carr ied ou t by measuring the sticking probabi l i ty S for hydrogen on clean and H2S-eovered thin iron fi lm surfaces by means of a f low m e t h o d [1, 2]. The inf luence of p readsorbed HaS adspeeies on the charac te r o f hydrogen deposi t was inves t iga ted with appl icat ion of TDS [1].

The exper iments were per formed using a glass U H V appara tus capable of reaching 10 -1~ Tor r rout inely. I ron wire (Johnson M a t t h e y grade I), wound a r o u n d a tungs ten heater was appl ied for thin iron fi lm deposi t ion on walls o f the cell ma in ta ined at 78 K. The films were s intered for .~ 30 min at ~ 350 K. Control led a m oun t of H2S was preadsorbed a t 298 K (in one case a t 195 K in .a T D S experiments) , and then the cell was evacuated . Hydrogen was adsorbed a t 78 K, af ter cooling down the cell.

In Fig. 1 are presented hydrogen TD spectra registered b y means of a mass spec t rome te r (Topatron, Leybold) f rom clean (a) and H2S preeovered (195 K, b) and (298 K, c ) t h in iron f i lm surfaces, 5.84X101~ H2S molecules/era 2 were preadsorbed in eve ry ease. The roughness fac tor of our fihns was 17 __ 1 [3].

t~emembering t h a t the average dens i ty of a toms on ~ th in iron fi lm surface is 1.63X 10 ~s [4], and H.2S occupies ~ sites [5], i t can be es t imated t h a t ~.10% o f to ta l num be r of accessible sites was preeovered.

77

NOWACKI et al.: THIN IRON FILMS

ffl

c

c~

z3

i

0~, ' ~ -~

0 r ' - ~

Tempero~ur% K

Fig. 1. I-Iydrogen TD spectra from thin iron films, clean (a) and preeovered with H~St at 195 K (b) and at 298 K (c)

I t can be seen in Fig. 1 that H2S preadsorbed under the conditions described above, does not change the main character of hydrogen deposit. Two desorption peaks, characterized by activation energies of desorption 90 kJ/mol and 67 kJ/mol were observed, similarly to previous reports for the hydrogen- iron system [3]. However, the amount of hydrogen adsorbed depends strongly on the character of HaS deposit. I-I2S preadsorbed at 195 K significantly dimi- nishes hydrogen adsorption. The ratio of the amount of hydrogen adsorbed on ir~on film precovered with I.I~S at 195 K to the amount adsorbed on a clean surface was found to be 0.5 and 0.4 on the basis of TDS and volumetric cal- culations, respectively. On the other hand, It2S preadsorbed at 298 K increases the amount of hydrogen consumed. The ratio mentioned above reaches now 1.7 and 1.6, respectively.

We suppose that molecularly adsorbed tteS poisons the thin iron film surface. for hydrogen adsorption, occupying the most active sites, while dissoeiatively adsorbed H2S, changing the character of hydrogen adatom polarization from electronegative to eleetropositive [6], promotes the incorporation of hydrogen below the surface. Drastic increase of permeability of iron foils for hydrogen in the presence of HaS [7] confirms this assumption.

The same conclusion can be drawn on the basis of sticking probabili ty (S), dependence on hydrogen population N H for clean and HaS precovered thin iron films (Fig. 2). In Fig. 2 curve A corresponds to hydrogen adsorption or~

78

NOWACKI et al.: THtN I R O N FILaMg

clean iron surface, while B and C are connected with adsorption on iron surface precovered with 2.49• H.~S molecules/cm '~ and 2.77X10 i4 H~S mole- eules/cm 2, respectively. One can notice that a small coverage of H~S such as 4 • -a (curve B Fig. 2) influences significantly the S (N~) behavior. The curve C in Fig. 2 resembles the S (N~) relation for the process of hydride for- mation associated with hydrogen incorporation below the surface [1, 8].

h

$

i ~ - \ . _

~d 3 I

4 i

i 4

A ~B C

-SL_.__ T ! I [ -,. ~0 0 i 2 4 5

Fig. 2. Sticking probability dependence on hydrogen population for adsorption on clean (A), and H2S preeovered thin iron films (C and ]3)

I t was interesting to examine the validity of Kisliuk's model [ 9] for adsorption with the precursor state for the curves A, B and C in Fig. 2, as was done pre- viously for the hydrogen-iron system [10]. According to this model [9]

S 1 - O - - - 0 0 2 ( ] ) S~ 1-t 1 - 0 K + I - ~ - O -S~

lim where O is hydrogen coverage, S O = 0 -~0 S, and K is a parameter independent

of O, showing the relation between the probability of adsorption, migration and desorption of the precursor state.

71L

NOWACKI et aL: THIN IRON FILMS

The question arises how to express O in the presence of H2S adspecies and hydrogen penetration below the surface. One can express 0 in two ways :

(i) 0 = NHmax(Fe)

Where NH max (~e) is the maximal uptake of hydrogen on a clean iron surface. With this relation for curves B and C, O can be higher than 1.

NH {ii) O~ =

Nltmax(z)

Where Nm~,~(z) is the maximal uptake of hydrogen in every individual experi- ment. The results are shown in Fig. 3 (parts I and II). Points correspond to the experimental results, lines are calculated using eq. 1. The continuous line cor- responds to adsorption on a clean iron surface, dashed lines to adsorption on the surface with preadsorbed It2S. Similarly as was found previously [10], hydrogen adsorption on a clean thin iron film surface fits Kisliuk's model well. In contrast to that, preadsorption of H2S at 298 K changes the character of the kinetics of hydrogen adsorption so strongly tha t eq. 1 no more describes this process.

t

-4, f

1o II- I Part '~

! _ _ i I I [ ~d 0.2 0.4 o.e o.8 to ~.2 i.4 1.6 L8

e

]

S

1(; 3

~5

~x, x

- ",',. \

"I Part I1

I 1 I t '1 0.2 0.4 0.6 0.8 7.0

e=

Fig. 3. Exanfination of Kisliuk's model for hydrogen adsorption on clean (continuous line) and ~[2S preeovered (dashed line) thin iron films for S (o) (Part I) and S (o z) (Part It). Points correspond to the experimental results

,~0

NOWACK[ et al.: THIN IRON FILMS

REFERENCES

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81 (1960). 5. I. E. Den Besten, P. W, Selwood: J. Catai, 1, 93 (1962). 6. I. Szymerska, W. Palczewska: Rocz, Chem., 45, 435 (1971). 7. W. Palczewska, I. 1%atajczykowa: Buil. Acad. Polon. Sci. Set. Sci. Chim., 9, 267

(~961). 8. P. Nowacki, 1%. Dug: 1%ocz. Chem., 51, 103 (1977). 9. P. Kis l iuk: J. Phys. Chem. Solids, 5, 78 (1958).

10. P. Nowacki, W. Lisowski, 1%. Dug: React . Kinet . Catal. Left . , 26, 297 (1984).

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